In typical acoustic investigations of earth formations from inside a borehole a tool is used with a sonic pulse generating transmitter and a plurality of sonic receivers. These are spaced from the transmitter and produce waveforms representative of the acoustic waves incident upon the receivers after having traveled through the earth formation and the borehole. The acoustic waves arriving at the receivers include distinctly detectable waves such as the compressional, shear, the direct or borehole fluid, and Stoneley, The Stoneley waves are deemed as late arrivals, i.e. waves whose velocity is less (or whose slowness is greater) than the direct or mud wave (see U.S. Pat. No. 4,131,875 to Ingram). The acoustic tool may be formed with a sonic pulse transmitter with many receivers such as four in the Ingram patent or twelve as described in an article entitled "Semblance Processing Of Borehole Acoustic Array Data" by Kimball and Marzetta and published in Geophysics Vol. 49, No. 3, March 1984.
Late arrival waves, such as zero order, low frequency Stoneley waves are boundary waves whose amplitudes in homogeneous and non-fractured earth formations exhibit little or no attenuation over the transmitter-receiver spacings normally encountered with acoustic borehole logging tools. Such late arrival waves, however, are known to be sensitive to variations in the borehole diameter, the permeability of the earth formation and the presence of fractures. Such sensitivity is revealed by a change in the amplitude of the late arrival wave.
For example an article entitled "Synthetic Microseismograms: Logging In Porous Formations," by J. H. Rosebaum and published in Geophysics Vol. 39, No. 1 (February, 1974) at pages 14-32 describes a theoretical technique using Stoneley waves to obtain an indication of permeability from a measurement of their attenuation. U.S. Pat. No. 4,432,077 and an article entitled "A Long Spaced Acoustic Logging Tool" by Williams et al and presented at the SPWLA twenty-fifth annual logging symposium held in June 10-13, 1984 describe a technique to obtain an indication of permeability by measuring amplitude loss or energy loss of the tube wave and using the ratio of these parameters as detected by two vertically spaced receivers located in a borehole. A standard well known variable density display (VDL) of detected acoustic waves is shown and an increased signal-to-noise ratio is obtained by real-time stacking of eight waveforms attributable to different transmitter pulses. E. A. Koerperich presented a paper published at pages 1199-1207 in the August, 1978 publication of the Journal of Petroleum Technology and entitled "Investigation of Acoustic Boundary Waves and Interference Patterns As Techniques for Detecting Fractures". In this article the use of low frequency Stoneley waves to identify fractures is suggested.
Although these techniques describe ways to select the late arrival from waveforms with particular energy displays as shown in the Ingram patent, a need exists to provide a technique for detecting or indicating and evaluating the presence of a fracture in a fast, clear and unambiguous manner while a borehole is being logged by an acoustic tool.